Various frequency components or various mode components of a signal transmitted in an optical fiber have different transmission speeds, which results in waveform distortion of the signal, and such phenomena are referred to as dispersion. The impact of the dispersion on the optical transmission causes intersymbol interference between data pulses. Therefore, dispersion compensation is necessary for ensuring transmission performance of a system. For a high speed optical transmission system in a speed of above 40 Gb/s or an optical transmission network desired to be dynamically configured, a solution of adaptively adjustable dispersion compensation is necessary. To implement the adaptive dispersion compensation system, a mechanism of dispersion detection and feedback control is necessary.
At present, there are primarily three solutions of adjustable dispersion compensation, i.e. dispersion compensation based on adjustable chirped optical fiber Bragg grating, pre-distortion at the transmission side, and equilibrium at the reception side.
Three solutions of dispersion detection in the above dispersion compensation systems are primarily described as follows.
In solution 1, the amount of dispersion is determined through comparison of phase difference between clock signals.
Referring to FIG. 1, a method for detecting dispersion disclosed in a paper entitled “Chromatic Dispersion Monitoring Technique Using Sideband Optical Filtering and Clock Phase-shift Detection”, Journal of Lightwave Technology (JTL), Vol. 20, No. 12, is shown. In this method, a Vestigial side band-Upper (VSB-U) signal or a Vestigial side band-Low (VSB-L) signal on the signal spectrum is filtered out via an optical filter 101 before the photoelectric conversion at the reception side, then the photoelectric conversion is performed on the VSB-U signal or VSB-L signal through a photoelectric converter 102. A clock signal is extracted by a clock recovery unit 103, and the phase information of the sideband signal is clarified. Subsequently, the amount of dispersion is determined through the comparison of phase difference between two clock signals corresponding to the sideband signal and a baseband signal respectively.
This solution requires two sets of high speed photoelectric conversion and processing configuration, and is complicated in the configuration. In addition, periodical repetition may occur to the clock phase difference, therefore, only the amount of dispersion corresponding to up to the range of one clock cycle may be measured, and the range of the amount of measurable dispersion is limited.
In solution 2, a transmission signal is added with a harmonics detection signal which is extracted after the photoelectric conversion at the reception side, and the amount of system dispersion is determined via the intensity of the harmonics detection signal at the reception side.
Referring to FIG. 2, a solution of dispersion monitoring and compensation using a single in-band sub-carrier tone, disclosed in WH4, OFC2001, is shown. In this solution, a harmonics detection signal is added, via a modulator, to a signal transmitted into a transmission line, spectrum power of the harmonics detection signal is separated by an electric filter after the photoelectric conversion of the optical signal at the reception side, and the amount of system dispersion is determined according to the decrease in the amount of power.
In this method, an additional tune device needs to be added at the transmission side, which increases the complexity of system.
In solution 3, an optical signal is converted to an electric signal, and the amount of system dispersion is determined by detecting a change of a first minimum point of dispersion in power spectrum.
A dispersion detection solution is disclosed in U.S. Pat. No. 6,487,352. As shown in FIG. 3, the implementation of this detection solution is as follows: After the photoelectric conversion of an optical signal, the signal is filtered via some RF narrowband filters, and then a value of the amount of system dispersion is determined through an analysis of several narrowband spectrum components. In this solution, correspondence relationship between the cosine expansion degree of the power spectrum of a square law receiver and the amount of signal dispersion is utilized. Here, the reference numbers in FIG. 3 are respectively as follows: 300 represents input optical signal; 301 represents photoelectric converter; 302 represents adjustable amplifier; 303 represents band-pass filter; 304 represents adjustable amplifier; 305 represents band-pass filter; 306 represents square detector; 307 represents low pass filter; 308 represents A/D converter; 309 represents digital signal processor; and 310 represents dispersion control signal.
In the above patent, the wave trough position of power spectrum is searched for by high-density sampling for the power spectrum, and the amount of dispersion of a signal is determined with the offset direction and offset amount of the wave trough position. The high-density sampling is necessary for positioning accurately the wave trough position, which complicates the configuration of the detection system, and increases the cost of the same. Further, even a tiny change from the amount of dispersion of a signal needs to be detected and compensated in a high speed system. However, the change of the wave trough position is not sensitive to a tiny variety in the amount of dispersion, which limits the application of this solution in the high speed system. In addition, in this solution, the electric power at each filter's output needs to be amplified, which increases power consumption of the system.